The word Teef refers to a persons big fat Gnashers or Dentures, normally the two biggest front teeth, also known as tombstones are targeted. If someone says you have big teef, it can be very insulting if used in a good sentence. Normally you would insult the largeness or goofyness of someones teef. Also see teefy weefy.

Dan: "Look at your big hairy teef Chris!"
Ian: "Chris has teefy tombstones"
Chris: "I may have big gappy teef, but you two have beaver gnashers"